It's definitely possible to drastically improves one's vert in their 20's. I'd been dunking since my freshman year of high school but didn't start really dunking (i.e. dunking frequently in games, sometimes on people) until I hit my 20's.

I say, if you want to become a higher jumper, do not become discouraged due to your age. When I graduated from high school, I strangely believed 18 years old would be my athletic peak. Instead, I think the average athletic peak is closer to 27. I'm 29 now and have been getting up pretty well for the past decade.

I think 2swift4u is actually right on point. If you've been maxing out your workouts for years (like, say, pounding through jump programs, playing hours per day, heavy/maximum training), maybe you've peaked and you cannot go any further. But in most cases, I think people have room for improvement.

My other thought process has been this: maybe I could dunk at age 18 without having worked out to my full potential. Perhaps I was only achieving 50% of my potential, yet it still yielded solid results. I think the affects of aging can sometimes be counteracted by working harder and better. For instance, maybe you'll have to work hard enough to achieve 70% of your potential by age 24, and more as you get older. And maybe if you work hard enough, the results may surpass what you were able to achieve at a younger age.